oh jeez looks like worm is eating away inside the one leaf. Try pull the damaged leaf off so the infestation doesn't spread. Neem oil is a good preventative to add into the feeding system. the plant will absorb it and make the leaf tastes nasty to any bugs that try it eat. We also used a organicide weekly to help stop bug infestations. Do your research to what you use though as some things are not meant to be used on marijuana plants as the plant will be smoked with the chemical you apply which can cause health issues.